In the Linux kernel, the following vulnerability has been resolved:
bpf: track changes_pkt_data property for global functions
When processing calls to certain helpers, verifier invalidates all packet pointers in a current state. For example, consider the following program:
__attribute__((__noinline__)) long skb_pull_data(struct __sk_buff *sk, __u32 len) { return bpf_skb_pull_data(sk, len); }
SEC("tc") int test_invalidate_checks(struct __sk_buff *sk) { int *p = (void *)(long)sk->data; if ((void *)(p + 1) > (void *)(long)sk->data_end) return TCX_DROP; skb_pull_data(sk, 0); *p = 42; return TCX_PASS; }
After a call to bpf_skb_pull_data() the pointer 'p' can't be used safely. See function filter.c:bpf_helper_changes_pkt_data() for a list of such helpers.
At the moment verifier invalidates packet pointers when processing helper function calls, and does not traverse global sub-programs when processing calls to global sub-programs. This means that calls to helpers done from global sub-programs do not invalidate pointers in the caller state. E.g. the program above is unsafe, but is not rejected by verifier.
This commit fixes the omission by computing field bpf_subprog_info->changes_pkt_data for each sub-program before main verification pass. changes_pkt_data should be set if:
- subprogram calls helper for which bpf_helper_changes_pkt_data
- subprogram calls a global function,
The verifier.c:check_cfg() pass is modified to compute this information. The commit relies on depth first instruction traversal done by check_cfg() and absence of recursive function calls:
- check_cfg() would eventually visit every call to subprogram S in a
- when S is fully explored:
- every direct helper call within S is explored
- every call to subprogram S1 called by S was visited with S1 fully
The downside of such approach is that dead code elimination is not taken into account: if a helper call inside global function is dead because of current configuration, verifier would conservatively assume that the call occurs for the purpose of the changes_pkt_data computation.